Comparative study of lipolysis by PLA2 of DOPC substrates organized as monolayers, bilayer vesicles and nanocapsules.

The water-soluble lipolytic enzymes act at the interface of insoluble lipid substrates, where the catalytical step is coupled with various interfacial phenomena as enzyme penetration, solubilization of reaction products, loss of mechanical stability of organized assemblies of phospholipids molecule, etc. One biologically relevant example is the enzymatic hydrolysis of DOPC by PLA(2), which results in cleavage of phospholipids molecules into water insoluble lipolytic products, namely oleic acid and lysophospholipid. In general, the enzymatic activity depends on the substrate organization and molecular environment of the catalytic reaction. The lipolysis by phospholipase A(2) of dioleoylphosphatidylcholine substrates organized as monolayer, bilayers vesicles and lipid nanocapsules was studied by measuring the decrease of the surface area at constant surface pressure or increase of the surface pressure at constant area at air-water interface. A kinetic model describing the coupling of the catalytic act with corresponding interfacial phenomena was developed. By using the kinetic model the values for the global hydrolytic kinetic constants were obtained. The obtained value for the monolayer is five orders of magnitude higher than this obtained with small unilamellar vesicles and six orders of magnitude higher then those obtained with lipid nanocapsules. The comparison shows that the enzymatic catalytic act occurring in the lipid environment of the monolayer is more efficacious than at the vesicle and nanocapsules interfaces.

Action of Humicola lanuginosa lipase on mixed monomolecular films of tricaprylin and polyethylene glycol stearate.

The hydrolysis catalyzed by Humicola lanuginosa lipase (HLL) of pure tricaprylin (TC) or stearate of polyethylene glycol 1500 (PEG-St) as well as their mixtures spread as monomolecular films were studied. The catalytic transformation of the two substrates TC or PEG-St into their respective reaction products was detected by measuring simultaneously the decrease in the film area and the surface potential using the "zero order" trough at constant surface pressure. A kinetic model describing the enzymatic hydrolysis was developed. The surface concentrations of the two substrates and their respective reaction products as well as the values of the global kinetic constants of hydrolysis were determined. The experimentally obtained global kinetic constants of the catalytic action of HLL against TC and PEG-St present in mixed monolayers of TC/PEG-St are approximately the same as in the case of pure monolayers. These obtained results give some indications that the activity of enzyme is not significantly affected by the different molecular environments in the mixed monolayers.

Interactions between hen egg-white lysozyme, PEG2,000, and PLA50 at the air-water interface.

In this paper, we compared the efficiency of polymer films, made of a poly(ethylene glycol) (PEG2,000)/poly(d,l-lactide) (PLA50) mixture, or a PEG2,000-PLA50 copolymer, to prevent adsorption of a model protein, the hen egg-white lysozyme (HEWL), at the air-water interface. This was achieved by analyzing the surface pressure/surface area curves, and the X-ray reflectivity data of the polymer films spread on a Langmuir trough, obtained in absence or in presence of the protein. For both the mixture and the copolymer, the amount of protein adsorbed at the air-water interface decreases when the density of the polymer surface coverage increases. It was shown that even in a condensed state, the polymer film made by the mixture can not totally prevent HEWL molecules to adsorb and penetrate the polymer mixed film, but however, protein molecules would not be directly exposed to the more hydrophobic phase, i.e. the air phase. It was also shown that the configuration adopted by the copolymer at the interface in its condensed state would prevent adsorption of HEWL molecules for several hours; this would be due in particular to the presence of PEG segments in the interfacial film.

Spectroscopic studies on poly(ethylene glycol)-lysozyme interactions.

In the present paper, different spectroscopic methods were applied to evaluate conformational changes of hen egg-white lysozyme (HEWL) in various solvents and in the presence of poly(ethylene glycol) (PEG). In citrate (0.007M, pH=6), or in Tris (0.1M, pH=7.4), no conformational change of the protein was measured across the range of concentrations tested. In addition, HEWL in ultra-pure water revealed no irreversible conformational change and no activity loss, at least at low concentrations (< or =0.2mg/ml). Whereas PEG can induce a reorganization of water molecules, no change of the secondary and tertiary protein conformations was observed in the presence of PEG. In addition, in the presence of PEG of various molecular weights, no change of enzymatic activity of the HEWL was observed across the range of concentrations tested.

Interfacial properties of adsorbed films made of a PEG2000 and PLA50 mixture or a copolymer at the dichloromethane-water interface.

Adsorption kinetics of films of poly(ethylene glycol) (PEG2000) studied by the dynamic pendant drop method showed that PEG2000 was more tensioactive at the dichloromethane (DCM)-water interface than at the air-water interface. When initially solubilized into DCM, PEG2000 segments would form an adsorbed layer with hydrophobic segments buried into the polymer chains turned toward the organic phase. Compression of this layer, accompanied by viscoelastic effects, led to expulsion of some hydrophilic tails toward the water phase. When initially dissolved in water, adsorption of PEG2000 segments led to an elastic PEG2000 layer organized on both sides of the interface. Results showed that when the PEG2000-PLA50 (poly(D,L-lactide)) copolymer film was adsorbed at the DCM-water interface, it resulted in a mixed layer exclusively turned toward DCM and its rheological properties were governed by PLA50. When adsorption at the DCM-water interface resulted from a physical mixture of PEG2000 and PLA50, rheological properties of the film were influenced by the initial localization of PEG2000 in the bulk phases. In the case of a mixed film formed by the adsorption of PLA50 from DCM and PEG2000 from water, results showed that PEG2000 segments totally pushed those of PLA50 away from the interface and exclusively influenced the behavior of the mixed film.

Properties of polyethylene glycol 660 12-hydroxy stearate at a triglyceride/water interface.

This study proposed a method to understand the surfactant role in the first step of the formulation of a novel generation of lipidic nanocapsules. A dynamic rheological protocol was applied using a pendant drop tensiometer in order to determine the interfacial properties of the initial mixture implied in the first formulation step. The response, in terms of interfacial elasticities, described how this mixture led to monodisperse nanometer size range structures after a physico-chemical constraint.

Influence of some formulation parameters on lysozyme adsorption and on its stability in solution.

According to our results concerning the behavior of lysozyme at interfaces, its secondary structure and its enzymatic activity, successful protein encapsulation would need to maintain a pH value far from the enzyme isoelectric point value during the formulation to reduce, in particular, the adsorption of lysozyme molecules at the created interfaces. Moreover, buffers or salt solution must be used in order to keep intact the native secondary conformation of lysozyme, and preserve its enzymatic activity.

Development and characterization of solid lipid nanoparticles loaded with magnetite.

This paper describes the preparation of colloidal lipid particles containing magnetite from warm emulsions. A two step method was used to obtain the nanoparticles: (i) formulation of a transparent phase by heating a O/W emulsion (aqueous surfactant solution melted with a lipid phase, containing the ethyl oleate and soybean lecithin) in which modified lipophilic magnetite is incorporated, and (ii) preparation of the nanoparticles by dispersing the warm transparent phase in cold water (7 degrees C) under mechanical stirring. The latter method gives spherical nanoparticles of a mean size of 62 nm measured by Photon Correlation Spectroscopy and Transmission Electronic Microscopy. The magnetite entrapment efficiency was determined by use of a magnetophoretic sedimentation method.

Interfacial properties of amiodarone: the stabilizing effect of phosphate anions.

Amiodarone, a drug used in heart therapy, is poorly soluble in water at room temperature, but forms transparent phases much more concentrated than the critical micellar concentration (CMC), when crystals are heated (above 60 degrees C) in presence of water and cooled down to room temperature. These pseudosolutions were supposed to be made of a complex system of micelles. In order to better understand the effects of pH and ion species on the supramolecular organization of amiodarone, interfacial pressure measurements were performed at the air/water interface on a Langmuir trough. Monolayers spread from chloroformic solutions over non bufferered subphases were insoluble at basic pH (NaOH, pH 10) but soluble at acidic pH (HCl, pH 4). However, a higher ionic strength obtained by adding NaCl (0.15 N) or NaH(2)PO(4) (0.15 N) to the subphase stopped the amiodarone solubilization. On an acidic phosphate subphase (NaH(2)PO(4), pH 4.4, 0.15 N), abnormally high surface pressures (>1 mN/m) were measured for high molecular areas (80-200 Å(2)/molecule) suggesting a supramolecular organization of the surface film. Insoluble monolayers were also obtained when the amiodarone supramolecular pseudosolution was spread on neutral (NaH(2)PO(4), pH 6.25, 0.15 N) or acidic (NaH(2)PO(4), pH 4.4, 0.15 N) subphases. However, a great instability on basic subphase (phosphate buffer pH 8.8) indicated the breakage of the supramolecular structure during spreading. These results are discussed taking into account the amiodarone state of ionization and the electrostatic interactions with counterions. Combining the use of phosphate counterions and that of acidic pH opens new perspectives in the optimization of amiodarone intravenous formulations.

Why does PEG 400 co-encapsulation improve NGF stability and release from PLGA biodegradable microspheres?

PURPOSE: The aim of this work was to understand the mechanism by which co-encapsulated PEG 400 improved the stability of NGF and allowed a continuous release from PLGA 37.5/25 microspheres. METHODS: Microparticles were prepared according to the double emulsion method. PEG 400 was added with NGF in the internal aqueous phase (PEG/PLGA ratio 1/1 and 1.8/1). Its effect was investigated through interfacial tension studies. Protein stability was assessed by ELISA. RESULTS: A novel application of PEG in protein stabilization during encapsulation was evidenced by adsorption kinetics studies. PEG 400 limited the penetration of NGF in the interfacial film of the primary emulsion. Consequently, it stabilized the NGF by reducing the contact with the organic phase. In addition, it avoided the NGF release profile to level off by limiting the irreversible NGF anchorage in the polymer layers. On the other hand, the amount of active NGF released in the early stages was increased. During microparticle preparation, NaCl could be added in the external aqueous phase to modify the structure of microparticles. This allowed to reduce the initial release rate without affecting the protein stability always encountered in the absence of PEG. CONCLUSIONS: PEG 400 appeared of major interest to achieve a continuous delivery of NGF over seven weeks from biodegradable microparticles prepared by the double emulsion technique.

Tensioactivity and supramolecular organization of the palmitoyl prodrug of timolol.

PURPOSE: To improve the bioavailability of the ocular drug timolol by facilitating its transport through the cornea, an amphiphilic prodrug was synthesized via the addition of a palmitic chain by esterification. The present study was undertaken to investigate the physicochemical and tensioactive properties of the prodrug. METHODS: The amphiphilic properties of the prodrug were firstly investigated by the Wilhelmy plate method. The textures generated by the supramolecular organizations of the ester were visualized by optical microscopy. RESULTS: The prodrug clearly decreased the surface tension. Optical microscopy provided excellent evidence for the existence of lyotropic liquid crystalline phases: two isotropic but organized phases and a birefringent lamellar phase. CONCLUSIONS: The results from the ensemble of studies undertaken to determine the amphiphilic properties of the prodrug were all in accord with its ability to form liquid crystalline phases. The liquid crystalline state of the prodrug is believed to introduce a delay in the drug pharmacological effect.

Surface characterization of poly(alpha-hydroxy acid) microspheres prepared by a solvent evaporation/extraction process.

This work constitutes the first attempt to characterize the wettability of poly(alpha-hydroxy acid) (PAHA) microspheres in situ, prepared according to a complex process involving emulsification, solvent evaporation, washing and freeze-drying. The analysis of the flotation profile of the microspheres has allowed us to determine both advancing and receding contact angles at the microsphere/air/water interface and furnished information on the organization of poly(vinyl alcohol) (PVA) and bovine serum albumin (BSA) at the surface of the PAHA coating. By the comparison of contact angles measured from model surfaces obtained by sampling pure PAHA, PVA, BSA and mixed PVA/PAHA monolayers on glass and poly(methyl methacrylate) (PMMA) substrates, it was concluded that the emulsifier (PVA or BSA) was strongly anchored to the surfaces of the microspheres. The use of BSA to formulate the microspheres from a single oil-in-water emulsion led to dry particles having a hydrophobic surface. The unfolding of the hydrophilic segments of the BSA embedded at the surface of the microspheres, following immersion in water, increased the wettability of the microspheres by water. The same qualitative results were obtained when PVA was used to stabilize single emulsions. On the other hand, microspheres formulated from a double water-in-oil-in-water emulsion displayed no modifications of their wettability when immersed in water. This can be explained by the absence of mobility of the hydrophilic segments of the emulsifier which are blocked in the surface or at the subsurface of the polymer matrix.

Kinetics of liposome disintegration from foam film studies: effect of the lipid bilayer phase state.

The kinetics of interfacial liposome breakdown is investigated in the thin liquid film microinterferometric set up of Scheludko et al. Suspensions of small unilamellar vesicles of dimyristoylphosphatidylcholine are studied at temperatures above and below the temperature of the main gel-liquid crystal first order phase transition. The experimentally established time traces of the velocity of thinning of foam films are used to estimate the kinetic (rate) constants of interfacial liposome disintegration. New and previously established data for other lipids are summarized and compared with results from kinetic measurements of lipid monolayer formation. The thin film experiments confirm the existence of interfacial liposomal aggregates. A change in the kinetic behaviour is observed, due to the 'melting' of the hydrophobic tails in the lipid aggregates. This may have various consequences of biological and pharmacological importance.

A structural study of interfacial phospholipid and lung surfactant layers by transmission electron microscopy after Blodgett sampling: influence of surface pressure and temperature.

Monolayer studies of the lung surfactant extract (LSE), dipalmitoyl phosphatidilcholine (DPPC) and dioleyl phosphatidilcholine (DOPC) have been performed in the dynamic condition at various temperatures. These compounds were also studied by differential scanning calorimetry, and the Langmuir Blodgett films were examined by electron microscopy. The combination of these techniques allowed us to describe precisely the collapse process, which was found to be different above and below the transition temperature of the lipids. However, whereas a phase separation for DPPC/DOPC mixtures occurred at all temperatures studied, this separation was observed for LSE only at temperatures lower than that characteristic of the "rigid state" to "liquid-like state" transition temperature. The ability of LSE to rapidly respread upon decompression appears to be due to the formation of piled amorphous aggregates formed during compression of its monolayers.

[Tension activity of pulmonary surfactant: adsorption of liposomes of model phospholipids]. / Tensioactivié du surfactant pulmonaire: adsorption de liposomes de phospholipides modèles.

One peculiarity of pulmonary surfactant, which is the tensioactive material physiologically present at the surface of alveoli, lies in its very quick localization at in the air-water interface. This being one of the limiting factors of artificial exogenous surfactants for the treatment of patients suffering of respiratory distress syndromes, we have studied the mechanisms which are intervening in the adsorption kinetics of a pure liquid--phase phospholipid, the dioleylphosphatidylcholine (DOPC), and of mixtures of dipalmitoylphosphatidylcholine (DPPC) and phosphatidic acid (PA) in presence of divalent cations (Ca++ and Mg++). The adsorption kinetics of liposomal suspensions of DOPC, which were studied by the Wilhelmy plate method, are determined by the existence of a barrier potential which height depends on the temperature and medium osmolarity, and on the deformability of vesicules. The study of PA-DPPC liposomes was performed with the help of a pulsating bubble surfactometer, a physicochemical instrumentation which mimics the pulmonary alveoli. To obtain performant responses with this model, high concentrations of PA and of divalent cations Ca++ and Mg++ are needed. These results, which are similar to those observed during the study of liposomal fusion, allow to propose a model, according to which adsorption of liposomes at the air-water interface is comparable to liposomal fusion and may be related to the presence of a thin aqueous film.

Wettability of polymers by mucin aqueous solutions.

The wettability of poly(methyl methacrylate) and polyethylene by water and aqueous mucin solutions have been studied by sessile drop and under-water captive air bubble contact angles, respectively. From the sessile drop and octane under-water contact angles the polymer-water interfaces have been characterized in terms of works of adhesion and acid-base (polar) interactions. A large water-air contact angle hysteresis observed with poly(methyl methacrylate) surfaces has been attributed to side-chain beta relaxations of polymer ester methyl groups. The wettabilities of the polymers by mucin aqueous solutions have been studied as a function of protein concentration and related to the surface tensions. A positive slope of adhesion tension vs surface tension line, characteristic of polar surfaces, was found with poly(methyl methacrylate). By contrast, a change in the slope, explained as a change in mucin relative adsorption densities at solid/liquid and solid/vapour interfaces, was observed with polyethylene. This adhesion tension behavior appeared to be in agreement with previous data we have published concerning the quantity and state of mucin which are adsorbed to polymers characterized by different surface properties.

[Problems posed by the development of exogenous surfactants for the treatment of hyaline membrane disease]. / Les problèmes posés par la mise au point de surfactants exogènes pour le traitement de la maladie des membranes hyalines.

The hyaline membrane disease (HMD) is a respiratory distress occurring at birth of some premature infants, attributed to an endogenous pulmonary surfactant deficiency. The present treatments are iatrogenic and inefficient for the most diseased infants. The exogenous surfactant supplementation intends to give to the baby formulations playing in vivo the role of the natural surfactant. The research developed for the formulation and the diffusion of these exogenous surfactants have to solve physicochemical problems but also of industrial production and of safety use. In spite of their adequate physicochemical and pharmacological properties, surfactants of natural sources, for industrial grounds, are not still distributed at a large scale. The efficiency of artificial surfactants is variable, and the use of non-biodegradable molecules in several preparations let to safety problems. The development of new types of artificial surfactants, more efficient and safe, implies a better knowledge of the physico-chemical mechanisms intervening in the pulmonary surfactant dynamics.

Collagen at interfaces. II: Competitive adsorption of collagen against albumin and fibrinogen.

Adsorption of chemically radiolabeled [14C] collagen from binary mixtures with albumin or fibrinogen was studied on the solution/air and solution/polyethylene interfaces and revealed the preferential adsorption of albumin. This phenomenon is confirmed by the data of surface tension measurements of single protein, collagen-albumin, and collagen-fibrinogen solutions. Desorption experiments clearly show that more irreversibly adsorbed collagen was found on polyethylene surfaces when adsorption was performed from collagen-fibrinogen than from collagen-albumin solutions. The combined adsorption-desorption and the surface tension data show that competitive adsorption of collagen at the hydrophobic surfaces is strongly influenced by the surface tension properties of the proteins in solution.

Collagen at interfaces. I. In situ collagen adsorption at solution/air and solution/polymer interfaces.

Collagen was isolated from rat tail tendons and acetylated with 1-14C acetic anhydride. In situ adsorption of this collagen from a buffer solution (pH = 2.7) was measured at the interfaces to air, polyethylene and polyethylene grafted with poly(maleic acid), respectively. The kinetics of adsorption were recorded for all surfaces studied and the corresponding diffusion coefficients for collagen in solution with various protein concentrations were calculated. The desorption of collagen from polymer surfaces was also studied. These experiments reveal the existence of both a reversibly and an irreversibly adsorbed collagen layer on the polymers tested. The desorption/adsorption ratio for the polyethylene is higher than that for the grafted polyethylene indicating stronger interactions of collagen with the grafted surface than with the non-modified polyethylene.

Adsorption of bovine submaxillary mucin on silicone contact lenses grafted with poly(vinyl pyrrolidone).

Bovine submaxillary mucin is considered to be an analogue of the high molecular protein present in the conjunctival mucus. This mucin was isolated from fresh salivary glands and acetylated with [1-14C]acetic anhydride. In situ adsorption of the bovine submaxillary mucin on silicone contact lenses ungrafted and grafted with poly(vinyl pyrrolidone) was performed for the first time using an original radiotracer technique. The results show that the adsorbed amounts of mucin are higher on grafted samples and that thick layers are adsorbed when mucin concentration in the bulk solution is increased. Desorption experiments reveal that in addition to the tightly adsorbed protein layer, a loosely bound mucin layer of the same thickness exists on grafted and ungrafted silicones.